1. Field of the Invention
The present invention relates to high-capacity, step-up DC/DC converters, and particularly to technologies for downsizing the same.
2. Description of the Related Art
FIG. 1 is a circuit configuration diagram of a conventional DC/DC converter. This DC/DC converter is described in JP2002-10632 A. The DC/DC converter shown in FIG. 1 is a step-up converter for large output current. In order to reduce the ripple current which flows through a smoothing capacitor C1, two converters are connected in parallel in the DC/DC converter, the converters operating with a 180° phase shift.
A switch Q1 comprised of a MOSFET or the like is connected to both ends of a DC power supply Vdc1 through a reactor L1. A switch Q2 comprised of a MOSFET or the like is connected to both ends of the DC power supply Vdc1 through a reactor L2. A series circuit including a diode D1 and the smoothing capacitor C1 is connected between a node to which the reactor L1 and the switch Q1 are connected, and a negative terminal of the DC power supply Vdc1. A series circuit including a diode D2 and the smoothing capacitor C1 is connected between a node to which the reactor L2 and the switch Q2 are connected, and the negative terminal of the DC power supply Vdc1. A load RL is connected to both ends of the smoothing capacitor C1.
The reactor L1, the diode D1, and the switch Q1 constitute a first converter. The reactor L2, the diode D2, and the switch Q2 constitute a second converter.
A control circuit 100 causes the high-frequency switching operation of the switches Q1 and Q2 with a 180° phase shift (half a cycle). The inductance value or the frequency are set so that the electric current flowing through the reactor L1 and the electric current flowing through the reactor L2 become zero every switching cycle.
Next, an operation of the conventional DC/DC converter thus configured will be described with reference to a timing chart of signals shown in FIG. 2.
At a time t30, once the switch Q1 is turned on by a Q1 control signal Q1g from the control circuit 100, an electric current flows through a path from a positive side of the DC power supply Vdc1, to the reactor L1, to the switch Q1, and then to a negative side of the DC power supply Vdc1. Accordingly, an electric current Q1i in the switch Q1 linearly increases. At the same time, an electric current L1i in the reactor L1 also linearly increases.
At a time t31, once the switch Q2 is turned off by a Q2 control signal Q2g from the control circuit 100, an electric current Q2i in the switch Q2 rapidly becomes zero. At this time, the energy stored in the reactor L2 is supplied to the load RL via the diode D2 and the smoothing capacitor C1. An electric current L2i in the reactor L2 also decreases from its peak value with a gradient corresponding to the difference value between the input voltage and the output voltage.
At a time t32, once the switch Q2 is turned on by the Q2 control signal Q2g from the control circuit 100, the electric current Q2i in the switch Q2 linearly increases. At the same time, the electric current L2i in the reactor L2 also linearly increases.
At a time t33, once the switch Q1 is turned off by the Q1 control signal Q1g from the control circuit 100, the electric current Q1i in the switch Q1 rapidly becomes zero. At this time, the energy stored in the reactor L1 is supplied to the load RL via the diode D1 and the smoothing capacitor C1. The electric current L1i in the reactor L1 also decreases from its peak value with a gradient corresponding to the difference value between the input voltage and the output voltage. The operation carried out at a time t34 is similar to that carried out at the time t30.
However, the DC/DC converter shown in FIG. 1 requires two reactors L1 and L2. In addition, if a perfect symmetry between the two converters inclusive of the circuit wiring cannot be maintained, the electric currents in these converters cannot be balanced, and the uneven loss will be caused. Moreover, a correction circuit or the like which carries out correction so that the electric currents in the converters are balanced, becomes necessary, which has resulted in a disadvantage that the circuit becomes complicated.
Furthermore, if the step-up ratio of the DC/DC converter is high, since conduction angles of the switching elements become large, conduction angles of the diodes D1 and D2 become small. Thus, peak current becomes high, and the ripple current which flows through the smoothing capacitor C1 therefore increases. As a result, the smoothing capacitor C1 has to be large.